1. Field of the Invention
The present invention relates generally to an exposure control apparatus in a photographic camera. More particularly, the invention relates to an exposure control apparatus in which the exposure time is varied as a function of the level of light received from a scene to be photographed and in which a low light signal is provided to indicate that the level of scene light may be too low for a proper film exposure.
2. Description of the Prior Art
Many relatively inexpensive cameras are available today having an exposure control apparatus in which the effective size of the exposure aperture is manually selected and the speed of the shutter is electronically determined as a function of the level of scene light. The electronic shutter in these "aperture preferred" cameras is connected to a light integrating timing circuit for determining the exposure time. Typically, the timing circuit is energized in synchronization with movement of a shutter opening blade to initiate an exposure interval by uncovering the exposure aperture. The timing circuit subsequently actuates a shutter closing blade to terminate the exposure interval by covering the exposure aperture. The closing blade is actuated after a time period established by the timing circuit in accordance with the level of scene light received by a photocell in the circuit. As is well known, the timing circuit can compensate for the particular size exposure aperture selected by adjusting an electrical parameter of the timing circuit to vary its time constant. Often, this adjustment is accomplished by attenuating the effective sensitivity of the photocell in proportion to the exposure aperture setting. This may be done, for example, by placing a neutral density filter or a partially obscuring mask in front of the cell surface.
It is common practice to include another photocell in the camera which receives light from the scene to be photographed. The function of this photocell is to control energization of an indicator lamp in the viewfinder. The indicator lamp is energized when the level of scene light is too low for a proper film exposure at the selected aperture without the aid of a camera support or an auxiliary flash source. In many cameras, the low light indication is provided only when the shutter speed corresponding to the selected aperture is 1/30 sec. or slower. Other cameras provide a low light indication only at a shutter speed of 1/60 sec. or slower. At 1/30 sec. and to a lesser degree at 1/60 sec. or, for that matter, at 1/90 sec., a user of the camera may not be able to hold it steady, with the result that the film image of the scene being photographed is smeared, causing an attendant loss of resolution in the photographic prints made from the film negative.
To reduce the space otherwise occupied in the camera by the separate exposure time determining and low light determining photocells, it is known to use a single photocell structure having two independant photoconductive portions. An example of such a structure is disclosed in U.S. Pat. No. 3,748,974, granted July 31, 1973 in the name of T. D. Cochran. With use of the single photocell structure, placing a neutral density filter or a partially obscuring mask in front of the cell surface to attenuate the effective sensitivity of the photocell will affect the low light signal as well as the shutter speed. Therefore, a low light signal will be provided only at those shutter speeds which are slower than a predetermined shutter speed. For example, a low light signal will be provided only at a shutter speed of 1/30 sec. or slower, or alternatively, only at a shutter speed of 1/60 sec. or slower. No provision is made, however, to change the shutter speed at which a low light signal will result. That is, the shutter speed which corresponds to a low light condition remains the same, regardless of the level of scene light or the particular size exposure aperture selected for use. This situation is depicted in FIG. 3 of the accompanying drawings. FIG. 3 is a diagram of an aperture preferred exposure program having two alternate exposure apertures, f/5.6 and f/9.5, and various shutter speeds ranging from 1/30 sec. to 1/250 sec. As shown in the diagram (which assumes a film ASA speed of 400) a low light signal is provided only at a shutter speed of 1/30 sec. or slower. In operation, the camera user manually sets the exposure aperture at f/9.5 when scene light is thought to be relatively bright. However, if this assessment of scene light is incorrect, that is, if scene light is 30 footlamberts or less, a low light signal is produced because the resulting shutter speed of f/9.5 for 30 footlamberts would be 1/30 sec. The low light signal is produced before the shutter opening blade can be released to uncover the exposure aperture and instructs the camera user to stop up to f/5.6. At f/5.6 the resulting shutter speed for 30 footlamberts would be 1/90 sec. Of course, when the camera user initially sets the exposure aperture at f/5.6 because scene light is thought to be relatively low, a low light signal is produced if scene light is 10 footlamberts or less, since the resulting shutter speed at f/5.6 for 10 footlamberts would be 1/30 sec. Therefore, with the exposure program of FIG. 3, a low light signal is provided only at a shutter speed of 1/30 sec. or slower, regardless of the level of scene light or the particular size exposure aperture selected.